The portable hole (DMG 264) (20,000 gp; 0 lbs.) doesn't adhere to the formulas on Table 7–33: Estimating Magic Item Gold Piece Values (285). The hole is a nonstandard legacy magic item dating back to at least the Dungeon Master's Guide (1977) for Advanced Dungeons & Dragons, wherein, for comparison, either the hole's finder could sell it for 50,000 gp and earn an equal number experience points or the hole's finder could keep it and earn 5,000 experience points.
How Dungeons and Dragons, Third Edition arrived at its 14,000 gp price and the 3.5 revision at its 20,000 gp price is a mystery. (However, I suspect in the former case the price was just set arbitrarily, but in the latter that price was picked because it's conveniently twice the price of the biggest bag of holding (DMG 248) (10,000 gp; 60 lbs.). This is pure speculation, though.)
In each of these three editions plus the one revision a portable hole is 6 ft. in diameter.
Creating a portable hole
Caster level usually isn't a prerequisite for the creation of many magic items, and caster level actually isn't a prerequisite for the creation of a portable hole. (See also answers to this question for a more detailed analysis.) So, while the typical portable hole is created at caster level 12, a portable hole can be created at a minimum caster level of 9 by a typical cleric, at a minimum caster level of 14 by a typical sorcerer, or at a minimum caster level of 13 by a typical wizard, each of whom use the 5th-level Clr spell and 7th-level Sor/Wiz spell plane shift [conj] (PH 262) to create the portable hole.
There's no reason a portable hole can't be made at caster level 9 by a cleric and such a hole will duplicate the effects of a portable hole created at caster level 20 by a sorcerer—except, for example, the sorcerer's portable hole will be harder to suppress for 1d4 rounds with an effect like the 3rd-level Sor/Wiz spell dispel magic [abjur] (PH 223). These decreases and increases in caster level neither arbitrarily change the hole's effect nor the hole's price! (That is, a creator that makes his own portable hole at caster level 20 simply benefits from having made the hole himself rather than buying an off-the-rack hole that was created at caster level 12.)
Creating a bigger portable hole
To create a bigger portable hole, a comparison can be made to the other extradimensional storage space that's like a portable hole, the relic the enveloping pit (Magic Item Compendium 159) (3,600 gp; 0 lbs.). However, the combination of the pit's 50-ft. depth, its relic power, and limited functionality in the hands of a misaligned creature probably—in this DM's opinion—shouldn't've resulted in a pit receiving a weird 82% discount over the price of a typical portable hole that subsequently makes the pit the best storage device for its price in the game… if the owner is the correct alignment and owns a really long ladder.
In other words, ask the DM how to create an even bigger portable hole. This DM and player suspects that were bigger holes easily and inexpensively created, rules for creating them would be printed somewhere, but I'm unaware of a bigger hole (besides the pit, of course) in the whole of the D&D 3e corpus. With the legacy issues the hole has1 and a lack of reasonable items with which to compare bigger holes to, a creator may have to content himself with multiple holes instead of one big hole.
1 The 4-volume Encyclopedia Magica for AD&D 2e that compiles every magic item published for the various D&D versions since the game's inception until 1994 and that has multiple entries for almost everything has in volume 3 but the lone entry for portable hole.